1. Field of the Invention
The present invention relates to a sintered silicon carbide. More particularly, it relates to a dense sintered silicon carbide having high strength obtained by a pressureless sintering as a normal sintering.
2. Description of the Prior Art
Silicon carbide is well known as a useful ceramic source having high hardness, excellent wear resistance, low thermal expansion coefficient, high decomposition temperature, high oxidation resistance and chemical resistance, and relatively high electric conductivity. A dense sintered silicon carbide has said characteristics and also has high strength even at high temperature, high heat shock resistance, and is considered to be effective as a source for high temperature structural products and to be used for various uses such as a gas turbine. Silicon carbide has relatively high covalent bond whereby it is difficult to sinter silicon carbide by itself. In order to obtain a dense sintered product, it is necessary to incorporate a certain sintering additive. In a hot press process, boron, B.sub.4 C, aluminum, AlN or Al.sub.2 O.sub.3 has been used as a sintering additive. In the prior art, for example, U.S. Pat. No. 3,836,673, a sintered silicon carbide having a strength of 10.sup.4 psi (70 kg/mm.sup.2) has been obtained by a hot press under an addition of 0.5 to 5 wt.% of aluminum. Thus, the use of metallic aluminum and the hot press process have the below-mentioned disadvantages.
In a pressureless sintering, it has been known to incorporate aluminum and carbon. In the prior art, for example, U.S. Pat. No. 4,280,497, a product having high flexural strength has been obtained by a pressureless sintering of a mixture of silicon carbide and 0.2 to 2 wt.% of aluminum component and 0.1 to 2.0 wt.% of carbon component. In this process, aluminum is mainly used and carbon is added for an acceleration of sintering. As the carbon source, a resin is used whereby a hardening of a resin causes trouble in the process. Moreover, the conventional pressureless sintered product has not been satisfactory in view of the characteristics and process.